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B::Deparse(3pm) Perl Programmers Reference Guide B::Deparse(3pm)

NAME

B::Deparse - Perl compiler backend to produce perl code

SYNOPSIS

ppeerrll -MMOO==DDeeppaarrssee[,,-dd][,,-ffFILE][,,-pp][,,-qq][,,-ll]

[,,-ssLETTERS][,,-xxLEVEL] prog.pl

DESCRIPTION

B::Deparse is a backend module for the Perl compiler that generates

perl source code, based on the internal compiled structure that perl

itself creates after parsing a program. The output of B::Deparse won't

be exactly the same as the original source, since perl doesn't keep

track of comments or whitespace, and there isn't a one-to-one corre-

spondence between perl's syntactical constructions and their compiled

form, but it will often be close. When you use the -pp option, the out-

put also includes parentheses even when they are not required by prece-

dence, which can make it easy to see if perl is parsing your expres-

sions the way you intended.

While B::Deparse goes to some lengths to try to figure out what your

original program was doing, some parts of the language can still trip it up; it still fails even on some parts of Perl's own test suite. If you encounter a failure other than the most common ones described in

the BUGS section below, you can help contribute to B::Deparse's ongoing

development by submitting a bug report with a small example. OOPPTTIIOONNSS As with all compiler backend options, these must follow directly after

the '-MO=Deparse', separated by a comma but not any white space.

-dd Output data values (when they appear as constants) using

Data::Dumper. Without this option, B::Deparse will use some simple

routines of its own for the same purpose. Currently, Data::Dumper is better for some kinds of data (such as complex structures with

sharing and self-reference) while the built-in routines are better

for others (such as odd floating-point values).

-ffFILE

Normally, B::Deparse deparses the main code of a program, and all

the subs defined in the same file. To include subs defined in other

files, pass the -ff option with the filename. You can pass the -ff

option several times, to include more than one secondary file. (Most of the time you don't want to use it at all.) You can also use this option to include subs which are defined in the scope of a

##lliinnee directive with two parameters.

-ll Add '#line' declarations to the output based on the line and file

locations of the original code.

-pp Print extra parentheses. Without this option, B::Deparse includes

parentheses in its output only when they are needed, based on the

structure of your program. With -pp, it uses parentheses (almost)

whenever they would be legal. This can be useful if you are used to LISP, or if you want to see how perl parses your input. If you say

if ($var & 0x7f == 65) {print "Gimme an A!"}

print ($which ? $a : $b), "\n";

$name = $ENV{USER} or "Bob";

"B::Deparse,-p" will print

if (($var & 0)) {

print('Gimme an A!') };

(print(($which ? $a : $b)), '???');

(($name = $ENV{'USER'}) or '???')

which probably isn't what you intended (the '???' is a sign that perl optimized away a constant value).

-PP Disable prototype checking. With this option, all function calls

are deparsed as if no prototype was defined for them. In other words,

perl -MO=Deparse,-P -e 'sub foo (\@) { 1 } foo @x'

will print sub foo (\@) { 1; } &foo(\@x); making clear how the parameters are actually passed to "foo".

-qq Expand double-quoted strings into the corresponding combinations of

concatenation, uc, ucfirst, lc, lcfirst, quotemeta, and join. For instance, print

print "Hello, $world, @ladies, \u$gentlemen\E, \u\L$me!";

print 'Hello, ' . $world . ', ' . join($", @ladies) . ', '

. ucfirst($gentlemen) . ', ' . ucfirst(lc $me . '!');

Note that the expanded form represents the way perl handles such

constructions internally - this option actually turns off the

reverse translation that B::Deparse usually does. On the other

hand, note that "$x = "$y"" is not the same as "$x = $y": the for-

mer makes the value of $y into a string before doing the assign-

ment.

-ssLETTERS

Tweak the style of B::Deparse's output. The letters should follow

directly after the 's', with no space or punctuation. The following options are available: CC Cuddle "elsif", "else", and "continue" blocks. For example, print if (...) { ... } else { ... } instead of if (...) { ... } else { ... } The default is not to cuddle. iiNUMBER Indent lines by multiples of NUMBER columns. The default is 4 columns. TT Use tabs for each 8 columns of indent. The default is to use

only spaces. For instance, if the style options are -ssii44TT, a

line that's indented 3 times will be preceded by one tab and

four spaces; if the options were -ssii88TT, the same line would be

preceded by three tabs. vvSTRING..

Print STRING for the value of a constant that can't be deter-

mined because it was optimized away (mnemonic: this happens when a constant is used in vvoid context). The end of the string is marked by a period. The string should be a valid perl

expression, generally a constant. Note that unless it's a num-

ber, it probably needs to be quoted, and on a command line quotes need to be protected from the shell. Some conventional

values include 0, 1, 42, '', 'foo', and 'Useless use of con-

stant omitted' (which may need to be -ssvv""''UUsseelleessss uussee ooff ccoonn-

ssttaanntt oommiitttteedd''.."" or something similar depending on your

shell). The default is '???'. If you're using B::Deparse on a

module or other file that's require'd, you shouldn't use a

value that evaluates to false, since the customary true con-

stant at the end of a module will be in void context when the file is compiled as a main program.

-xxLEVEL

Expand conventional syntax constructions into equivalent ones that expose their internal operation. LEVEL should be a digit, with

higher values meaning more expansion. As with -qq, this actually

involves turning off special cases in B::Deparse's normal opera-

tions.

If LEVEL is at least 3, "for" loops will be translated into equiva-

lent while loops with continue blocks; for instance

for ($i = 0; $i < 10; ++$i) {

print $i;

} turns into

$i = 0;

while ($i < 10) {

print $i;

} continue {

++$i

}

Note that in a few cases this translation can't be perfectly car-

ried back into the source code - if the loop's initializer

declares a my variable, for instance, it won't have the correct scope outside of the loop. If LEVEL is at least 5, "use" declarations will be translated into "BEGIN" blocks containing calls to "require" and "import"; for instance, use strict 'refs'; turns into sub BEGIN { require strict; do {

'strict'->import('refs')

}; } If LEVEL is at least 7, "if" statements will be translated into equivalent expressions using "&&", "?:" and "do {}"; for instance

print 'hi' if $nice;

if ($nice) {

print 'hi'; }

if ($nice) {

print 'hi'; } else { print 'bye'; } turns into

$nice and print 'hi';

$nice and do { print 'hi' };

$nice ? do { print 'hi' } : do { print 'bye' };

Long sequences of elsifs will turn into nested ternary operators,

which B::Deparse doesn't know how to indent nicely.

UUSSIINNGG BB::::DDeeppaarrssee AASS AA MMOODDUULLEE SSyynnooppssiiss

use B::Deparse;

$deparse = B::Deparse->new("-p", "-sC");

$body = $deparse->coderef2text(\&func);

eval "sub func $body"; # the inverse operation

DDeessccrriippttiioonn

B::Deparse can also be used on a sub-by-sub basis from other perl pro-

grams. nneeww

$deparse = B::Deparse->new(OPTIONS)

Create an object to store the state of a deparsing operation and any options. The options are the same as those that can be given on the

command line (see "OPTIONS"); options that are separated by commas

after -MMOO==DDeeppaarrssee should be given as separate strings. Some options,

like -uu, don't make sense for a single subroutine, so don't pass them.

aammbbiieennttpprraaggmmaass

$deparse->ambientpragmas(strict => 'all', '$[' => $[);

The compilation of a subroutine can be affected by a few compiler directives, pprraaggmmaass. These are: +o use strict; +o use warnings;

+o Assigning to the special variable $[

+o use integer; +o use bytes; +o use utf8; +o use re;

Ordinarily, if you use B::Deparse on a subroutine which has been com-

piled in the presence of one or more of these pragmas, the output will include statements to turn on the appropriate directives. So if you then compile the code returned by coderef2text, it will behave the same way as the subroutine which you deparsed.

However, you may know that you intend to use the results in a particu-

lar context, where some pragmas are already in scope. In this case, you use the aammbbiieennttpprraaggmmaass method to describe the assumptions you wish to make.

Not all of the options currently have any useful effect. See "BUGS" for

more details. The parameters it accepts are: strict Takes a string, possibly containing several values separated by whitespace. The special values "all" and "none" mean what you'd expect.

$deparse->ambientpragmas(strict => 'subs refs');

$[ Takes a number, the value of the array base $[.

bytes utf8 integer If the value is true, then the appropriate pragma is assumed to be in the ambient scope, otherwise not.

re Takes a string, possibly containing a whitespace-separated list of

values. The values "all" and "none" are special. It's also permis-

sible to pass an array reference here.

$deparser->ambientpragmas(re => 'eval');

warnings

Takes a string, possibly containing a whitespace-separated list of

values. The values "all" and "none" are special, again. It's also permissible to pass an array reference here.

$deparser->ambientpragmas(warnings => [qw[void io]]);

If one of the values is the string "FATAL", then all the warnings in that list will be considered fatal, just as with the wwaarrnniinnggss pragma itself. Should you need to specify that some warnings are fatal, and others are merely enabled, you can pass the wwaarrnniinnggss parameter twice:

$deparser->ambientpragmas(

warnings => 'all', warnings => [FATAL => qw/void io/], ); See perllexwarn for more information about lexical warnings. hintbits warningbits These two parameters are used to specify the ambient pragmas in the

format used by the special variables $^H and ${^WARNINGBITS}.

They exist principally so that you can write code like:

{ my ($hintbits, $warningbits);

BEGIN {($hintbits, $warningbits) = ($^H, ${^WARNINGBITS})}

$deparser->ambientpragmas (

hintbits => $hintbits,

warningbits => $warningbits,

'$[' => 0 + $[

); } which specifies that the ambient pragmas are exactly those which are in scope at the point of calling. ccooddeerreeff22tteexxtt

$body = $deparse->coderef2text(\&func)

$body = $deparse->coderef2text(sub ($$) { ... })

Return source code for the body of a subroutine (a block, optionally preceded by a prototype in parens), given a reference to the sub. Because a subroutine can have no names, or more than one name, this

method doesn't return a complete subroutine definition - if you want

to eval the result, you should prepend "sub subname ", or "sub " for an anonymous function constructor. Unless the sub was defined in the main:: package, the code will include a package declaration.

BUGS

+o The only pragmas to be completely supported are: "use warnings",

"use strict 'refs'", "use bytes", and "use integer". ($[, which

behaves like a pragma, is also supported.) Excepting those listed above, we're currently unable to guarantee

that B::Deparse will produce a pragma at the correct point in the

program. (Specifically, pragmas at the beginning of a block often appear right before the start of the block instead.) Since the effects of pragmas are often lexically scoped, this can mean that the pragma holds sway over a different portion of the program than in the input file.

+o In fact, the above is a specific instance of a more general prob-

lem: we can't guarantee to produce BEGIN blocks or "use" declara-

tions in exactly the right place. So if you use a module which

affects compilation (such as by over-riding keywords, overloading

constants or whatever) then the output code might not work as intended. This is the most serious outstanding problem, and will require some help from the Perl core to fix.

+o If a keyword is over-ridden, and your program explicitly calls the

built-in version by using CORE::keyword, the output of B::Deparse

will not reflect this. If you run the resulting code, it will call

the over-ridden version rather than the built-in one. (Maybe there

should be an option to aallwwaayyss print keyword calls as "CORE::name".)

+o Some constants don't print correctly either with or without -dd.

For instance, neither B::Deparse nor Data::Dumper know how to print

dual-valued scalars correctly, as in:

use constant E2BIG => ($!=7); $y = E2BIG; print $y, 0+$y;

+o An input file that uses source filtering probably won't be deparsed

into runnable code, because it will still include the uussee declara-

tion for the source filtering module, even though the code that is produced is already ordinary Perl which shouldn't be filtered again. +o Optimised away statements are rendered as '???'. This includes

statements that have a compile-time side-effect, such as the

obscure

my $x if 0;

which is not, consequently, deparsed correctly.

+o There are probably many more bugs on non-ASCII platforms (EBCDIC).

AUTHOR Stephen McCamant , based on an earlier version by Malcolm Beattie , with contributions from Gisle Aas, James Duncan, Albert Dvornik, Robin Houston, Dave Mitchell,

Hugo van der Sanden, Gurusamy Sarathy, Nick Ing-Simmons, and Rafael

Garcia-Suarez.